Process for making a printing master

ABSTRACT

A PROCESS FOR MAKING A PRINTING MASTER WHEREIN A PHOTOCONDUCTIVE LAYER IS SELECTIVELY CHARGED TO A DESIRED AREA CONFIGURATION. THE CHARGED LAYER IS THEN IMMERSED IN AN EMULSION OF A CONDUCTING CONVERSION SOLUTION IN A NONCONVERTING NON-CONDUCTING LIQUID. IF THE SOLUTION IS OPPOSITELY CHARGED FROM THE AREA CONFIGURATION IT IS ATTRACTED TO THE CHARGED AREAS AND CONVERTS THEM WHILE THE NONCHARGED AREAS REMAIN UNCONVERTED. IF THE SOLUTION IS OF THE SAME CHARGE AS THE CHARGED AREAS, IT WILL BE REPELLED BY THEM AND THEY WILL REMAIN UNCONVERTED AND THE NONCHARGED AREAS, AFTER A PERIOD OF TIME, WILL BE CONVERTED.

United" States Patent 3,758,330 PROCESS FOR MAKING A PRINTING MASTER Edward B. Richman, Shaker Heights, Ohio, assignor to Addressograph-Multigraph Corporation, Cleveland, Ohio No Drawing. Filed Oct. 26, 1971, Ser. No. 192,584 Int. Cl. B41n 3/00; G03g 13/22 US. Cl. 117-37 LE 7 Claims ABSTRACT OF THE DISCLOSURE A process for making a printing master wherein a photoconductive layer is selectively charged to a desired area configuration. The charged layer is then immersed in an emulsion of a conducting conversion solution in a nonconverting non-conducting liquid. If the solution is oppositely charged from the area configuration it is attracted to the charged areas and converts them while the noncharged areas remain unconverted. If the solution is of the same charge as the charged areas, it will be repelled by them and they will remain unconverted and the noncharged areas, after a period of time, will be converted.

BACKGROUND OF THE INVENTION This invention relates generally to the making of printing masters, and more particularly to a process of making a printing master wherein a material which is normally oleophilic/hydrophobic is selectively electrostatically charged and the material is then contacted with a fluid which will be attracted to, and will convert only the areas to be altered in oleophilic nature. Usually it will be preferred to render the larger background area hydrophobic to reject ink and produce a printed image on an unaltered stock background.

One common prior art practice utilized an electrostatic process for making printing masters. In this process material which is normally oleophilic/ hydrophobic but which can be converted to hydrophilic/oleophobic condition is charged on the surface thereof and then selectively discharged leaving a charge only in the desired printing areas. The material is then contacted with a toner which itself is oleophilic/hydrophobic and which is not converted to hydrophilic/oleophobic condition. The toner is then fused by heat and/or pressure. Thereafter the material is immersed in a conversion solution which converts the nonimaged untoned area to a hydrophilic/oleophobic condition while the toner and the material underlying the toner and protected thereby remain oleophilic/hydrophobic. The master is then able to print using a grease or oil base ink in a conventional manner.

While this process does produce a good printing master, nevertheless it does require three separate steps to form the printing master after the image has been formed by electrostatic charge thereon; i.e. after the image is electrostatically formed on the material the material must be toned, the toner fused, and then the background area converted. Such three step process is time consuming and requires a substantial amount of equipment and controls therefor.

SUMMARY OF THE INVENTION According to the present invention a method of producing a printing master is provided wherein the oleophilic/ hydrophobic material is selectively charged to the desired image configuration and the material is then contacted with an emulsion of electrically conducting conversion fluid droplets in a non-conducting non-converting fluid which will selectively convert the desired area by electrical interaction of the charged area with conversion fluid particles.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is directed toward a simplified process of making a printing master by electrostatic processing. According to the present invention a material is provided which is normally oleophilic/hydrophobic in nature but which can be converted to hydrophilic/oleophobic in nature. The material also must be capable of being charged on the surface thereof and selectively discharged to provide a desired configuration of electrically charged and noncharged areas. Zinc oxide is the most common material which meets these requirements and hence constitutes the preferred base material. However, other materials can also be used.

According to the present invention, the zinc oxide material is provided on a substrate which can either be in the form of a drum or a sheet or strip of material. The zinc oxide is imaged in a conventional manner by first electrostatically charging the entire surface thereof and then exposing the areas which it is not desired to convert to a light source which will discharge the surface which is struck by the light. This will then provide a surface configuration of electrically charged and uncharged areas.

The zinc oxide is then contacted with an emulsion of a conversion solution emulsified and dispersed in a nonconverting non-conducting liquid with a low dielectric constant. With an emulsion of this nature the fine droplets of the conversion solution are attracted by the electrical charge to those areas which have an electrical charge thereon and the conversion solution when contacting these areas converts the material from oleophilic/hydrophobic to hydrophilic/oleophobic in nature. The fine droplets, however, are not attracted electrically to the non-imaged, i.e. non-charged, areas since there is no charge thereon to cause such attraction. Thus there will be very little contact of the fine droplets with the non-charged area. Hence the non-charged areas will not be converted whereas the charged area will be.

After the conversion of the charged area the material is removed from the conversion solution and it is ready for printing. If conventional oil base or grease base inks are used the ink will cause the non-converted areas to print, whereas the converted areas will not print. However, if conventional exposure techniques are utilized it is the background areas which are remaining oleophilic/hydrophobic and with conventional printing techniques using conventional grease or oil base inks the background areas would therefore print leaving the imaged areas unprinted. There are several techniques available in order to provide a non-printing background and a printing image. One such technique is to provide a fountain of oil, rather than water against the conventional material and utilize a water base ink. This would cause the imaged areas to print, but the oil in the background would prevent the background area from pickup of ink and printing. Such oils should be non-destructive to the material and have a high enough flash point to be safe. There are various hydrocarbons and mineral oils which are satisfactory.

Still another technique is the so called reversal process in converting the selectively charged photoconductive material. In this technique a conversion fluid which is electrically repelled by the charged area is utilized in the emulsion. In this case the conversion fluid will contact and convert the non-charged areas while the charged areas repel the conversion fiuid and remain unconverted.

Another technique is to first form a reverse image negative and then use such negative as the imaging device for selectively charging the zinc oxide. The thus imaged zinc oxide would then provide charged background and an uncharged image area, which would cause the background to be converted and the image area to be non-converted resulting in printing of the image and nonprinting of the background.

There are many combinations of liquid emulsion which will selectively convert the charged areas according to this invention. The requirements are that a conversion solution be capable of being attracted to an electrical charge while the liquid in which it is immersed be essentially non-conductive with a low dielectric constant. In this respect, in order to obtain best results the electrical resistance of the non-conducting fluid should be ohm-cm. or greater and the dielectric constant should be lower than 3.4.

There are many fluids which will satisfy these conditions. Preferably, however, the conversion solution is either an aqueous ferrocyanide type or an aqueous phosphate type and the insulating liquid is a hydrocarbon. A typical formulation for the ferrocyanide type of conversion is as follows:

Ingredient: Parts by weight Water 75 Mono-ammonium phosphate 8 Potassium ferrocyanide 1.5 Glycerin 15 A typical formulation for the phosphate type conversion solution is:

Ingredient: Parts by weight Water 75 Mono-ammonium phosphate 10 Phosphoric acid 1 Glycerin 15 For the insulating liquid many hyrocarbon liquids are satisfactory. For example, either Isopar H sold by Humble Oil and Refining Comapny or Sovasol 35 sold by Mobil Oil Company are very satisfactory as an insulating liquid in which the conversion solution can be emulsified.

Preferably the size of the droplets of the conversion solution in the emulsion should be between .5 and 500 microns, ideally about 10 microns. Such an emulsion can be satisfactorily formed and maintained using ultrasonic vibration. It has been found when utilizing a ferrocyanide or phosphate type conversion solution in either Isopar H or Sovasol 35 power generation of 400 watts at kilocycles is sufiicient for establishing and maintaining the emulsion. Also, if desired, a non-conducting surface active agent such as Span 85, manufactured by Atlas Powder Company, can be added in the amount of about 1 part per 1000 of emulsion to aid in stabilizing the dispersion although such agents are not necessary.

The exact proportions of the conversion solution and the insulating liquid may vary widely. The basic requirement in this respect is merely that the drops be sufficiently fine and sufficiently dispersed so that they will be attracted to the charged areas of the zinc oxide but they will not be in sufficient quantity to substantially convert any of the non-charged areas. For this situation the electrical resistance of the resulting emulsion should be about 10 ohm-cm. or greater. In the case of ferrocyanide or phosphate type conversion solutions in either Isopar H or Savasol 35, it has been found 1 to 10 parts conversion solution to 100 parts of insulating liquid by volume provides very satisfactory results. This formulation, however, is merely illustrative and is not intended to be a limitation.

What is claimed is:

1. In a method for preparing a lithographic printing master on an electrically insulating, oleophilic/hydrophobic surface, comprising the steps of electrostatically charging portions of said surface in accordance with a predetermined pattern to provide a surface having charged and uncharged areas, and converting one of said charged and uncharged areas to provide it with an oleophobic/hydrophilic surface, the improvement comprising contacting said charged and uncharged areas with an emulsion in which (a) the discontinuous phase is a liquid which selectively is deposited on one of said charged and uncharged areas, depending on whether it is attracted to or repelled by the electrostatic charge carried by said charged area, and converts the selected area to provide it with an oleophobic/hydrophilic surface, and

(b) the continuous phase is an insulating liquid having an electrical resistance of at least 10 ohm-cm, and a dielectric constant of less than about 3.4.

2. The method defined in claim 1 in which said discontinuous phase liquid is repelled by said charged area thereby converting said uncharged area to provide it with an oleophobic/hydrophilic surface.

3. The method defined in claim 1 in which said discontinuous phase is attracted to the electrostatic charge carried by said charged area thereby converting said charged area to provide it with an oleophobic/hydrophilic surface.

4. The method defined in claim 3 wherein the active ingredient in said discontinuous phase liquid is selected from the group consisting of aqueous ferrocyanides and aqueous phosphates and said continuous phase liquid is a hydrocarbon.

5. The method defined in claim 4 wherein said emulsion has an electrical resistance at least as high as 10 ohm-cm.

6. The method defined in claim 4 wherein the proportions of the liquids forming said emulsion are about 1 to about 10 parts by volume of discontinuous phase liquid to about parts by volume of continuous phase liquid.

7. The invention defined in claim 4 wherein the discontinuous phase liquid is in the form of droplets measuring within the range of about 0.5 to about 500 microns in diameter.

References Cited UNITED STATES PATENTS 2,957,765 10/ 1960 Resetich 961 R 2,988,988 6/1961 Kurz 96l R 3,329,499 7/1967 Garrett et a1 96-1 R 3,350,202 10/1967 Silver 96-33 X 3,677,756 7/1972 Protzman 96-33 X 3,432,406 3/1969 Eastman 204'18 PC 3,311,490 3/19'67 Fauser et a1. 25262.1 X

FOREIGN PATENTS 267,804 7/1964 Australia 1l737l.5 1,002,061 8/1965 Great Britain 252-62.1

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 

